1. Field of the Invention
The present invention generally relates to the fabrication of microstructures, such as integrated circuits, and, more particularly, to the incorporation of material species, such as dopants, into a substrate by ion implantation.
2. Description of the Related Art
In many technical fields, implantation of ions into a substrate is a widely used technique to alter the characteristics of the substrate or of a specified portion thereof by incorporating a specified species. For example, the rapid development in the semiconductor industry is among others based on the capability of advanced implantation techniques to generate highly complex dopant profiles within tiny regions of semiconductive and dielectric materials. In implanting specified ions into a substrate, not only a lateral implant profile may be readily obtained by providing correspondingly adapted implantation masks, for example formed as resist masks by photolithography, but also allows a vertical profile. To this end, an acceleration energy of the ions to be implanted is adjusted so as to deposit the majority of the ions at a specified depth of the substrate. Thus, contrary to doping a substrate by diffusion, lateral and in depth varying profiles may be created, thereby enabling the formation of complex dopant profiles as required, for instance, in well and drain and source structures of advanced transistor elements. Moreover, by appropriately selecting the dose, i.e., the number of ions per unit area of the ion beam impinging on a substrate, comparably high concentrations of atoms with short time intervals may be incorporated into a substrate compared to other techniques, such as diffusion, and therefore renders ion implantation a cost-effective technique.
Although ion implantation offers a plurality of advantages over other techniques, there are several drawbacks involved in employing ion implantation. For instance, the highly energetic ions generate severe damage in crystalline structures that commonly has to be cured by annealing the substrate, thereby changing the dopant profile as deposited owing to the unavoidable diffusion process during the anneal process. Similarly, the highly energetic ions may not be completely confined so as to exclusively hit the target, but may also interact with surface portions of the implantation tool, such as process chamber walls, inner surfaces of beam line, support means for fixing the substrate, and the like. During this interaction, metal atoms may be liberated and may become ionized and conveyed to the substrate where the metal atoms may be incorporated into the substrate.
A further source of contamination is the cross-contamination of the implantation tool and thus of a substrate processed therein. Cross-contamination may readily occur when an implantation tool is operated with a first dopant species, which may according to the implantation kinetics also interact with some surface portions of the tool that are exposed to the ions of the first species. Accordingly, ions of the first species may be incorporated into those exposed portions. Thereafter, the implantation tool may be used to implant a second, different species into the same or a different type of substrate, wherein then the atoms of the first species may be released by getting sputtered off the exposed surface portions. These atoms may be ionized and may then be deposited on and in the substrate along with the ions of the second species. In modem integrated circuits, however, even minute amounts of dopant variations may lead to significant variations of device characteristics due to the extremely complex dopant profiles required to adjust the properties of transistors having minimum feature sizes of 0.1 μm and even less.
Especially in sophisticated CMOS techniques, the well and the so-called halo implants require sufficiently steep dopant profiles to obtain the desired transistor performance. That is, usually heavy dopant species, such as arsenic, indium or antimony are used at high dosages in the range of approximately 1012 to 1014 ions/cm2 so as to confine the dopants within the region of interest with a desired small tolerance as, typically, heavy ions tend to get less scattered and thus less spread during the implantation process. Moreover, subsequent anneal processes affect the profile less as deposited, as is the case for lighter atoms. However, the heavy ions provided at a high dose may lead to an increased cross-contamination, especially when the implantation tool is used for the implantation of light ions, such as boron and phosphorus, at low energy as is required for source and drain implants and the like. Exclusively employing a specified tool with a certain dopant species may, however, not be considered an attractive option owing to cost concerns.
Therefore, a need exists for an effective technique that reduces cross-contamination during ion implantation when changing the dopant species.